The present invention relates to a process of treatment of a steel cord adapted for the reinforcement of rubber products, wherein the steel cord comprises steel filaments.
Steel cords are widely known and used for the reinforcement of rubber products such as tires, conveyor belts, timing belts and hoses.
In order to fulfill their reinforcement function in a proper way, the steel cords must have a high tensile strength, a sufficient resistance to compression once embedded in rubber, a good fatigue and corrosion resistance, a sufficient adhesion to rubber and a high impact resistance.
In addition to the above `first` type of properties, which all relate to the behaviour of steel cord in a rubber matrix, the rubber industry, and more particularly the tire industry, requires the steel cords to have still further properties. These additional properties, in contrast with the first properties, relate to the processability of the steel cords when handling the steel cords during the manufacture of, for example tires. Examples of these `second` type of properties include absence of residual torsions, straightness, absence of flare, etc.
Both the first and second types properties are supposed to be fulfilled between narrow specified limits. Conventionally, it is a difficult task, if not a rather impossible one, for a cord manufacturer to meet all these requirements.
Manufacturing a cord having all the desired properties is thwarted by the fact that, during the process of manufacturing of a steel cord, the result of measures taken to meet one property is wholly or partially nullified by measures taken in order to meet another property in the downstream part of the manufacturing process. The consequence is that a compromise must be sought or that one or more properties must be sacrified in favour of the other properties. PA0 For example, part of the tensile strength of the steel filaments gets lost during the subsequent twisting process. PA0 As another example, part of the degree of preforming may be lost during the downstream false twisting or during the downstream straightening. PA0 It is an object of the present invention to facilitate the meeting of some steel cord properties. PA0 It is another object of the present invention to make the attainment of some steel cord properties independent of the attainment of other steel cord properties. PA0 It is still another object of the present invention to provide for a process of manufacturing a high-elongation steel cord which has a controllable amount of residual torsions and which has a controllable openness and a sufficient and controllable elongation. PA0 It is a further object of the present invention to provide for a process of manufacturing a steel cord which has a controllable amount of residual torsions and has a sufficient and controllable openness and a sufficient and controllable P.L.E. (part load elongation, for definition see below). PA0 The process comprises PA0 Plastically deforming the steel filaments by overtwisting the steel cord means that the degree of overtwisting is such that the steel filaments are deformed beyond their elastical limit. Plastically deforming the steel filaments by over-twisting may decrease the radius of curvature of the steel filaments in a controllable way. The higher the degree of overtwisting in the plastical region, the smaller the radius of curvature. PA0 A small radius of curvature, if small enough, corresponds to an open cord structure, which allows rubber penetration even when the cord is put under a tensile tension of about twenty to fifty Newtons. This openness at tensile torsions between twenty and fifty Newton may be quantified by a P.L.E.-value in the case of single-strand constructions of the type 1 xn. P.L.E. means part load elongation and is the elongation of the cord at a predetermined tensile tension between twenty and fifty Newtons. PA0 In the case of high-elongation steel cords, the degree of plastical deformation of the steel filaments determines to a large extent the total elongation at fracture of the steel cord. PA0 The subsequent and downstream steps (iii) and (iv) are done in order to bring the number of residual torsions to zero or to a predetermined value. PA0 The advantage of the present invention is that during the steps (iii) and (iv) the steel cord is not stretched in such a way that the small radius of curvature of the steel filaments is increased again. This is reached by untwisting the steel cord under a low tension, i.e. a tension which is less than half the overtwisting tension. In other words, the plastical deformation of the filaments is not substantially modified during the steps (iii) and (iv). PA0 Steps (iii) and (iv) may be done by means of a second false twister.